MOCVD reactors are commonly used to grow epilayers of various metals with sharp interfaces between the epilayers. Normally, MOCVD reactors are oriented in the vertical direction. The reactor has a reactor chamber with an inductively or radiantly heated susceptor. The susceptor is placed with the deposition surface perpendicular to the flow axis of reactant gases through the reactor. The susceptor is often rotated slowly to minimize non-uniform heating effects. Reactants and carrier gases are typically introduced at the top of the reactor and flow down towards the hot susceptor. Due to the expansion of the gases with increasing temperature in approaching the hot susceptor, the reactor leads to destabilizing gas density gradients in the flow of reactant gases. In principle however, a uniform film thickness is deposited on the substrate of the susceptor. The film is made up of molecules of the desired metal of the reactant gas. It is desirable to provide sharp interfaces between the layers of deposited metal. These very thin even layers on the substrate are then used for various applications in the electronic industry. Normally, the metals and the reactant gases are introduced into the reactor chamber as metal-organic gases. The gases are heated to 700.degree. to 800.degree. C. to burn off the organic components with the result that a pure layer of metal is deposited on the wafer surface. The deposited layers of metal thus form what is called an epitaxial layer, that is the deposited metals have the same chemical lattice structure as the substrate. This imparts a property to the interface that causes electrons to behave in unusual ways and it is this property that is exploited in a variety of electronic devices.
The problem experienced with existing types of MOCVD reactors is the formation of gas vortices near the susceptor that cause uneven distribution of the epitaxial layers. These vortices are a product of the inherent gas velocity and the gas density gradients developed by the high temperatures as the gas approaches the hot susceptor. These difficulties with existing MOCVD reactors are discussed in detail in Fotiadis, D. I. et al, "Complex Flow Phenomena in Vertical MOCVD Reactors: Effects on Deposition Uniformity and Interface Abruptness" Journal of Crystal Growth 85:154-164 (1987). A Variety of MOCVD reactor configurations are investigated to determine the effect of the reactor configuration on the flow pattern of the reactant gases at various resident times. It is suggested that the inlet of the reactor may be packed with metal screens to achieve uniform inlet flow for the reactor. However, the sudden expansion of the flow into the enlarged cross-section of the reactor chamber creates large recirculation cells or vortices above the susceptor. Furthermore, the use of screens do not appreciably alter the flow characteristics of the incoming reactant gases.
Consideration has also been given to control of the flow of reactant gases in other types of reactors for coating particles. For example, in Lackey, W. J. et al, "Improved Gas Distributor for Coating High-Temperature Gas-Cooled Reactor Fuel Particles" Nuclear Technology Vol 35:227-237 (September, 1977) the investigation of various types of porous carbon plates as particularly mechanically treated to provide multiple blind holes are investigated. The advantage of this type of frit is to even out the flow of gases into the fluidized bed of particles. However, as discussed in this article, one of the disadvantages of this type of frit is its high cost of manufacture as well as the problem of the frit clogging with components of the reactants which are introduced to the fluidized bed reactor. Furthermore, it is not clear as to why a frit made of porous carbon and not any other material (stainless steel or incalloy for example) is preferred for the coating process. A possible reason is that, since the coating material is either carbon or silicon carbide (a small amount of carbon is usually present in most gas phase derived silicon carbide), carryover of carbon particles from the porous frit into the region of coating to a small extent perhaps does not affect the quality of the product. However, in the case of a MOCVD reactor for GaAs epilayer growth, one cannot tolerate the carryover of carbon particles (from the porous carbon frit) into the growth area as even ppm levels of carbon on the epilayer can cause a considerable loss in desired properties.